Resuscitation treatments for patients suffering from cardiac arrest generally include clearing and opening the patient's airway, providing rescue breathing for the patient, and applying chest compressions to provide blood flow to the victim's heart, brain and other vital organs. If the patient has a shockable heart rhythm, resuscitation also may include defibrillation therapy. The term basic life support (BLS) involves all the following elements: initial assessment; airway maintenance; expired air ventilation (rescue breathing); and chest compression. When all three [airway breathing, and circulation, including chest compressions] are combined, the term cardiopulmonary resuscitation (CPR) is used.
There are many different kinds of abnormal heart rhythms, some of which can be treated by defibrillation therapy (“shockable rhythms”) and some which cannot (non-shockable rhythms”). For example, most ECG rhythms that produce significant cardiac output are considered non-shockable (examples include normal sinus rhythms, certain bradycardias, and sinus tachycardias). There are also several abnormal ECG rhythms that do not result in significant cardiac output but are still considered non-shockable, since defibrillation treatment is usually ineffective under these conditions. Examples of these non-shockable rhythms include asystole, electromechanical disassociation (EMD) and other pulseless electrical activity (PEA). Although a patient cannot remain alive with these non-viable, non-shockable rhythms, applying shocks will not help convert the rhythm. The primary examples of shockable rhythms, for which the caregiver should perform defibrillation, include ventricular fibrillation, ventricular tachycardia, and ventricular flutter.
After using a defibrillator to apply one or more shocks to a patient who has a shockable ECG rhythm, the patient may nevertheless remain unconscious, in a shockable or non-shockable, perfusing or non-perfusing rhythm. If a non-perfusing rhythm is present, the caregiver may then resort to performing CPR for a period of time in order to provide continuing blood flow and oxygen to the patient's heart, brain and other vital organs. If a shockable rhythm continues to exist or develops during the delivery of CPR, further defibrillation attempts may be undertaken following this period of cardiopulmonary resuscitation. As long as the patient remains unconscious and without effective circulation, the caregiver can alternate between use of the defibrillator (for analyzing the electrical rhythm and possibly applying a shock) and performing cardio-pulmonary resuscitation (CPR). CPR generally involves a repeating pattern of five or fifteen chest compressions followed by a pause during which two rescue breaths are given.
Defibrillation can be performed using an AED. The American Heart Association, European Resuscitation Council, and other similar agencies provide protocols for the treatment of victims of cardiac arrest that include the use of AEDs. These protocols define a sequence of steps to be followed in accessing the victim's condition and determining the appropriate treatments to be delivered during resuscitation. Caregivers who may be required to use an AED are trained to follow these protocols.
Most automatic external defibrillators are actually semi-automatic external defibrillators (SAEDs), which require the caregiver to press a start or analyze button, after which the defibrillator analyzes the patient's ECG rhythm and advises the caregiver to provide a shock to the patient if the electrical rhythm is shockable. The caregiver is then responsible for pressing a control button to deliver the shock. Following shock delivery, the SAED may reanalyze the patient's ECG rhythm, automatically or manually, and advise additional shocks or instruct the caregiver to check the patient for signs of circulation (indicating that the defibrillation treatment was successful or that the rhythm is non-shockable) and to begin CPR if circulation has not been restored by the defibrillation attempts. Fully automatic external defibrillators, on the other hand, do not wait for user intervention before applying defibrillation shocks. As used below, automatic external defibrillators (AED) include semi-automatic external defibrillators (SAED).
Both types of defibrillators typically provide an auditory “stand clear” warning before beginning ECG analysis and/or the application of each shock. The caregiver is then expected to stand clear of the patient (i.e. stop any physical contact with the patient) and may be required to press a button to deliver the shock. The controls for automatic external defibrillators are typically located on a resuscitation device housing.
AEDs are typically used by trained medical or paramedic caregivers, such as physicians, nurses, emergency medical technicians, fire department personnel, and police officers. The ready availability of on-site AEDs and caregivers trained to operate them is important because a patient's chances of survival from cardiac arrest decrease by approximately 10% for each minute of delay between occurrence of the arrest and the delivery of defibrillation therapy.
Trained lay caregivers are a new group of AED operators. For example, spouses of heart attack victims may become trained as lay caregivers. Lay caregivers rarely have opportunities to defibrillate or deliver CPR, and thus they can be easily intimidated by an AED during a medical emergency. Consequently, such lay providers may be reluctant to purchase or use AEDs when needed, or might tend to wait for an ambulance to arrive rather than use an available AED, out of concern that the lay provider might do something wrong.
Some trained medical providers, e.g., specialists such as obstetricians, dermatologists, and family care practitioners, also rarely have the opportunity to perform CPR and/or defibrillate, and thus may be uneasy about doing so. Concerns about competence are exacerbated if training is infrequent, leading the caregiver to worry that he or she may not be able to remember all of the recommended resuscitation protocol steps and/or their correct sequence.
Similarly, both medical and lay caregivers may be hesitant to provide CPR and rescue breathing, or may be unsure when these steps should be performed, particularly if their training is infrequent and they rarely have the opportunity to use it.
It is well known to those skilled in the art, and has been shown in a number of studies, that CPR is a complex task with both poor initial learning as well as poor skill retention, with trainees often losing 80% of their initial skills within 6-9 months. It has thus been the object of a variety of prior art to attempt to improve on this disadvantageous condition. Aids in the performance of chest compressions are described in U.S. Pat. Nos. 4,019,501, 4,077,400, 4,095,590, 5,496,257, 6,125,299, and 6,306,107, 6,390,996. U.S. Pat. Nos. 4,588,383, 5,662,690 5,913,685, and 4,863,385 describe CPR prompting systems. AEDs have always included voice prompts as well as graphical instructions on flip charts or placards since the earliest commercial versions in 1974, to provide both correct timing and sequence for the complex series of actions required of the rescuer as well as placement of the defibrillation electrodes. U.S. patent application Ser. No. 09/952,834 and U.S. Pat. Nos. 6,334,070 and 6,356,785 describe defibrillators with an increased level of prompting including visual prompts either in the form of graphical instructions presented on a CRT or on printed labels with backlighting or emissive indicia such as light emitting diodes. AEDs since the 1970s have used the impedance measured between the defibrillation electrodes to determine the state of the AED as well as appropriate messages to deliver to the rescuer (e.g., “Attach Electrodes” if the initial prompts on the unit have been delivered and the impedance remains greater than some specified threshold; or to determine if there is excessive patient motion as in U.S. Pat. No. 4,610,254). U.S. Pat. No. 5,700,281 describes a device which uses the impedance of the electrodes to determine the state of the AED for delivering such messages as “Attach Electrodes.”
Enhanced prompting embodied in these patents provides some benefit to the rescuer in improved adherence to the complex protocol required of them to successfully revive a cardiac arrest patient, but it has been discovered in testing of the AEDs generally employing elements of these patents that rescuers are still only able to achieve a performance level of less than about 50%. The methods of the study were as follows: None of the subjects had prior experience or training with an AED in order to eliminate the potential for bias due to previous AED training. The test subjects were presented with a simulated use scenario more accurately resembling than in previous studies what a lay rescuer would encounter in a cardiac arrest rescue situation. Four fully-functional defibrillators were used: Physio-Control LifePak CR Plus, ZOLL AED Plus, the Philips/Laerdal HeartStart OnSite, and the Cardiac Science PowerHeart. The test subjects were led into a simulated office, and told that a person, simulated by a manikin, had just fallen to the floor, appeared to be completely unconscious and could well be dying. They were told to use the AED and any other object in the office and act as if it were a real emergency. Each person was evaluated based on the number of actions taken that comprise the Chain of Survival Sequence (8 steps: check response, seek help, open airway, check breathing, give breathes, check circulation, remove clothing, and attach AED electrodes). It was found that the Medtronic (Minnesota) Lifepak CR Plus group, which comprised 11 lay rescuers, averaged 3.5±1.4 steps completed; the Cardiac Science (California) PowerHeart group, which comprised 11 lay rescuers, averaged 3.4±1.9 steps; the Philips (Massachusetts) HeartStart group, which comprised 12 lay rescuers, averaged 3.8±1.3 steps; and the ZOLL (Massachusetts) AEDPlus group, which comprised 11 lay rescuers, averaged 5.0±1.3 steps completed. Even the ZOLL device that was shown to be statistically better than the other devices only achieved a 63% compliance rate. Further, less than 10% of the test subjects were able to sustain the recommended 100 compressions per minute for at least one minutes' duration.
It has recently been recognized that good CPR is essential to saving more victims of cardiac arrest (Circulation. 2005; 111:428-434). In the cited study, researchers found that in 36.9% of the total number of segments, compression rates were less than 80 compressions per minute (cpm), and 21.7% had rates of less than 70 cpm. The compression rate recommended by the American Heart Association in their guidelines is greater than 100 cpm. In the study, higher chest compression rates were significantly correlated with initial return of spontaneous circulation (mean chest compression rates for initial survivors and non-survivors, 90±17 and 79±18 cpm, respectively; P=0.0033). Further, this study was performed using well-trained rescuers, including nurses and physicians, indicating that the problem of poor compression rates is widespread.
AEDs with CPR feedback such as those of ZOLL and Philips mentioned above have some form of compression rate prompting. This takes the form of a beep or tone at the desired rate of 100 compressions per minute as recommended by the American Heart Association guidelines. The ZOLL AEDPlus has the added feature that it will begin the compression rate tones at the rate that the rescuer begins their compressions, and then gradually increases the compression tone rate up to the desired rate of 100 cpm. In some cases, this approach may be helpful, but because the compression tone rate is asynchronous with the rescuer's compressions, the tones may occur out of phase with the rescuer compression rate, and may actually act to confuse the rescuer and momentarily slow them down.
AEDs have also been solely focused on defibrillation, which, while it provides the best treatment for ventricular fibrillation and certain tachycardias, is of no therapeutic benefit for the 60% of the cardiac arrest patients presenting in pulseless electrical activity (PEA) or asystole. As AEDs are becoming more prevalent in the home, there are also a host of other health problems that occur such as first aid as well as incidents related to chronic conditions such as asthma, diabetes or cardiac-related conditions for which the AED is of no benefit.
After a defibrillation shock, the heart is in one of two states: either the shock was successful and the heart is in a stunned, ischemic condition with very little myocardial ATP energy reserves necessary for rhythmic pacemaker activity and effective cardiac output, or the shock was unsuccessful. Surprisingly to some, a defibrillation rarely, if ever, converts ventricular fibrillation into a normal sinus rhythm with effective hemodynamic output. Good CPR is required after a successful defibrillation shock in order for a patient to survive.
Although automated chest compression devices, such as that described by U.S. Pat. No. 6,752,771 have been synchronized with the cardiac cycle, rescuers providing manual CPR generally compresses the chest at a fixed rate with no synchronization to the cardiac cycle of a damaged heart such as occurs with pulseless electrical activity (PEA). PEA is a condition where the heart is functioning electrically, but does not have enough healthy muscle fibers to contract effectively. Patients typically have a very low ejection fraction where most of the blood in the heart remains in the ventricles during the contraction rather than being ejected in to the aorta and coronary arteries.
Many studies have reported that the discontinuation of chest compressions, such as is commonly done for ECG analysis, can significantly reduce the recovery rate of spontaneous circulation and 24-hour survival rate. These studies include “Adverse effects of interrupting precordial compression during cardiopulmonary resuscitation” by Sato et al. (Critical Care Medicine, Volume 25(5), May 1997, pp 733-736); “Adverse Outcomes of Interrupted Precordial Compression During Automated Defibrillation” by Yu et al. (Circulation, 2002); and “Predicting Outcome of Defibrillation by Spectral Characterization and Nonparametric Classification of Ventricular Fibrillation in Patients With Out-of-Hospital Cardiac Arrest” by Eftestøl et al. (Circulation, 2002).
In the context of automatic, mechanical compression systems, it has long been recognized that there are beneficial effects of synchronizing cardiac compression and ventilation cycles to the cardiac cycle. M. R. Pinsky, “Hemodynamic effects of cardiac cycle-specific increases in intrathoracic pressure”, Journal of Applied Physiology (Volume 60(2), pages 604-612, February 1986). U.S. Pat. Nos. 4,273,114, 4,326,507, and 6,752,771 describe mechanical compression systems that synchronize the compression cycle to the cardiac cycle. U.S. Patent application 2004/0162587 describes a mechanical compression system that modifies the chest compression based on monitored blood perfusion.
In U.S. Pat. No. 4,491,423 a resuscitation assistive timer is described that provides an audible compression rate that is adjusted based on the patient's age.